Techly explains: What causes that unique rain smell after weeks of no rain?

You know the smell – even if the only way you can describe it is “the smell of rain” – that beautiful, earthy aroma that hits your nostrils just before the downpour you’ve been waiting weeks for. It’s a smell we Aussies love, so it should be no surprise that a couple of scientists working in Australia were the ones to find out where it comes from.

In the 1960s, scientists Isabel ‘Joy’ Bear (an Aussie) and Richard Thomas (a Briton) – working out of the CSIRO’s Division of Mineral Chemistry in Melbourne – discovered that the smell in fact comes from an oil that seeps out of plants during periods without any rain.

The oil then makes its way to the ground, where clay-based rocks and soil soak it up.

Before rain hits, humidity increases, causing the tiny pores in the rocks and soil to fill with water, seeing the oil flushed out and into the air.

According to the CSIRO, Joy and Richard were able to find the “yellowish” oil by “steam distilling rocks that had been exposed to warm, dry conditions”.

On March 7, 1964, Nature published a paper authored by the CSIRO boffins, entitled “Nature of Argillaceous Odour”. In the paper, they named the smell “petrichor” – an amalgam of petra (meaning ‘stone’) and ichor (the blood of the gods in Greek mythology) – meaning ‘blood of the stone’.

The diverse nature of the host materials has led us to propose the name “petrichor” for this apparently unique odour which can be regarded as an “ichor” or “tenuous essence” derived from rock or stone.

As a result, not only has the odour become known as petrichor, so too has the oil which produces the scent.

Interestingly enough, the word ‘nyimpe’ means ‘the smell of rain’ in the indigenous Arrernte language in Alice Springs, so the idea has been around to thousands of years.

Joy and Richard continued their research, publishing a paper the following year entitled “Petrichor and Plant Growth”, in which they noted that petrichor slows seed germination, probably to stop them from attempting to grow in unfavourable conditions.

More recently, in January this year scientists at the Massachusetts Institute of Technology used high-speed cameras to create a video of petrichor being released into the air. They noted, “when a raindrop hits a porous surface, it traps tiny air bubbles at the point of contact. As in a glass of champagne, the bubbles then shoot upward, ultimately bursting from the drop in a fizz of aerosols.”

In discussing his findings, Cullen R. Buie – an assistant professor of mechanical engineering at MIT – talked about how Joy and Richard had discovered the oil, but not how it was released into the air.

“They talked about oils emitted by plants, and certain chemicals from bacteria, that lead to this smell you get after a rain following a long dry spell,” Buie said. “Interestingly, they don’t discuss the mechanism for how that smell gets into the air. One hypothesis we have is that that smell comes from this mechanism we’ve discovered.”

The MIT team also discovered why the petrichor smell isn’t as strong in heavier rain:

“When moderate or light rain hits sandy or clay soils, you can observe lots of aerosols, because sandy clay has medium wetting properties,” said Youngsoo Joung, a postdoctoral researcher in Buie’s lab.

“Heavy rain [has a high] impact speed, which means there’s not enough time to make bubbles inside the droplet.”

So there you have it, next time the sky turns grey after a long spell without so much as a drizzle, don’t ask “Can you smell the rain?”

Instead, ask “Can you smell the petrichor?”

About the author

Joe was Junior Vice-President at Compu-Global-Hyper-Mega-Net until it was bought out by Bill Gates. He now subedits for Conversant Media and considers it a step up.